This invention relates to novel phosphonium salts. The phosphonium salts of this invention are useful as initiators/catalysts in the reaction of epoxy resins with aromatic carbonate and/or ester linkages.
Conventionally cured epoxy resins contain residual secondary hydroxyl groups derived from oxirane ring opening by active hydrogens characteristic of such cured epoxy resins. The presence of such residual hydroxyl groups in conventionally cured epoxy resins impairs the performance and properties of the cured resin by providing a site for binding of water which acts to plasticize the cured epoxy resin, thus lowering tensile and flexural strength, lowering modulus, and increasing the dielectric constant. When oxirane groups are converted by reaction with carbonate or ester groups, no alcoholic hydroxyls are generated and the reaction can therefore be utilized to make epoxy-derived thermosets which absorb less water than conventionally cured epoxies. Also, such thermosets, when prepared from carbonates, possess higher cross-link densities than conventionally cured epoxies, since each carbonate group reacts with two oxirane moieties. Consequently, the epoxy/carbonate/ester cured product possesses improved temperature performance, increased water and solvent resistance, and improved dimensional stability.
The reaction of oxirane groups with aromatic carbonate and/or ester linkages requires use of an initiator/catalyst. Known types of initiators/catalysts found to be effective for this reaction typically show activity at relatively low temperatures of about 70.degree. to about 125.degree. C. It has been found that in many cases, particularly when the compound containing aromatic carbonate and/or ester linkages is a polymer such as polycarbonate, polyester, or polyestercarbonate, the above mentioned initiators/catalysts do not provide a sufficient processing window for melt blending the initiator/catalyst and the epoxy resin with the compound containing carbonate and/or ester linkages.
What is needed is an active and effective initiator/catalyst for the reaction of oxirane groups in epoxy resins with aromatic carbonate and/or ester linkages whose activity begins at a temperature of about 125.degree. C. or greater. Furthermore, the initiator/catalyst activity should be substantially developed below the decomposition temperature of the epoxy/carbonate/ester composition, which begins at about 300.degree.-320.degree. C. in air. The initiator/catalyst desirably does not initiate or speed up side reactions to such an extent that there does not result a cured product with a better combination of physical properties vis a vis the same epoxy resin per secured with generally comparable types of initiators/catalysts.